Do you have anything against nactating this play as D? It seems more natural to me.
No, nothing against it. D is fine. In a position where you are entering a checker, you have more options. You can either count the entering portion, or not count it (i.e., you can "waive" it), leaving D to describe the rest of the play.
Why did I use S (Split) then? It is out of habit as much as anything else. I'll explain how it fits into my paradigm in a moment.
First, as you pointed out, Z (reverse split) is also fine here. S has the arguable advantage of suggesting the word "Split" more instantly. And S works, because one cannot enter with the 6, nor for that matter come down with the 1; there is only one way to split. In short, either S or Z is fine. (If the 19pt and 12pt were both vacant, S would be a different play: Bar/19 13/12.)
Consider the positions below.
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There is no difference Nactation-wise between the two positions, but there may be a subtle difference in perception. When the 23pt is vacant, this "feels" more like a split (and down) type play. The reason is that in the left-hand position Bar/23 looks like it anchors rather than that it splits (or scoots) a checker. Nactation cannot descriptively afford to make a distinction such as anchoring or not anchoring with half the play because it only uses one character.
[Granted, @ for anchor is an option for the left-hand position that could be used assumptively, trusting that the reader will choose 13/8 over 8/3, but the assumption will be weaker or non-existent in similar positions and in any case it isn't entirely unambiguous/bulletproof.]
Now let's shift White's 5pt to her 4pt, as in the positions below.
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My point is that if you use D in the original position, you will need to be prepared in related positions to either walk through the hit/most/6 hierarchy and figure out which play is D and which is d, or switch back to S or Z anyway.
I don't mean to say that you don't have the right to prefer D in a position (such as Leobueno's 31P-43Z-53H-61) where there is no need to apply D/d hierarchy (and indeed it may be a more comprehensible choice for beginning interpreters); I'm merely explaining the reason I've gotten into the habit of using S or Z: I tend to nactate the entire play (rather than waive the entering portion) when I find it easy to do so. I tend to treat 21$-43 played 24/20* 13/10 the same way — S for completeness rather than D (by assumption) or H (by down default).
While on topic, I'll address Petter's comment as well. It is better as soon as possible to disabuse oneself of the notion that S might refer to a play like Bar/18, either for 61 in the original position, or with 52 in any of the four positions above, or say for opening 51 played 24/18 (which is 51U). "S" stands for split to remind you of the general concept but in Nactation the precise meaning tells us that you move a back checker somewhere on the far side and come down with the other move portion (even including the situation where you have a checker on the 21pt and 65S means 21/10). It is not meant to exactly reproduce the way that you might tend to use the word split for all positions in a chouette discussion. Bar/18 (in the first four positions) or 23/18 (in the two positions below, or in a variant of the left-hand position where Blue's 21pt checker is back on the 23pt anchored) does not come down with a move portion so it cannot be S.
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Right. But as long as you've brought up the issue, I am on the verge of making one final (though minor) change, which is that the 18pt will no longer be an overlap point for U and R. U will have the 24pt through 18pt destinations, and R will have the 17pt through 13pt destinations (or, as it has always been, R is for the entire far side when there is a move portion in both areas). I'm in the process of checking a bunch of positions, but I am already pretty sure this is a change for the better. [This may affect a few of your file names -- those with 51R played 24/18 or 52R or 61R played Bar/18 (sorry), but don't bother changing them to U — you'll know what R means anyway because it is the most runnish play available in those situations.]
Nack